Waterjet Technology – Cutting Plywood and Pork, and water jet safety

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

In the last two posts I have tried to show that there is a benefit to running an occasional calibration test on equipment to ensure that it is giving the best performance. This does not mean that the nozzle needs to be tested every day, although some of the cheaper pressure washer nozzles, for example, will wear out in less than an hour. An operator will learn over time about how long a nozzle will last and can, after a while, tell when it is starting to lose performance. But in working on a number of different jobs in succession, that sense of the performance may be missed, and it can be handy to have a standard target that a jet can be pointed at and that it should be able to cut in a known time.

One simple target is plywood, and, to continue the saga of nozzle comparisons through a slightly different approach, Mike Woodward used plywood sheets to compare different nozzles in one of the earliest comparisons of performance. We since duplicated his test equipment and ran tests with a more modern selection of nozzles but the basic results and conclusions remain the same.

In its simplest form, the idea is to build a holding frame that will hold small squares of plywood at fixed distances from the nozzle. In the frame shown below, the plywood pieces are set at one-foot distances apart with the nozzle held at a fixed point at the end of the test frame. Tests showed that it takes around 2,700 psi to cut through the plywood.

A simple frame to hold plywood samples

Figure 1. A simple frame to hold plywood samples

The initial tests that Dr. Woodward ran were run on nozzles that were run at 10,000 psi with a nominal flow rate of 10 gpm. The nozzles that were used cost in the range from $10.00 to $250 a piece. (And these costs were reported in 1985 at the 3rd American Waterjet Conference). Tests such as this are simple to run. Plywood pieces are set into the frame, the nozzle is placed at the end of the frame and the jet run for ten seconds. Over that time, the jet will cut through any of the pieces of plywood that it reaches with enough power to cut through, and generally, the jet will punch a hole through several pieces.

The different designs of nozzle that Mike Woodward tested in 1985

Figure 2. The different designs of nozzle that Mike Woodward tested in 1985

The profiles show that there was only one of the common nozzles at the time that fitted smoothly onto the end of the feed pipe. In the other cases, there is a small gap between the nozzle piece and the feed tube so that turbulence would be generated just as water entered the acceleration section of the nozzle.

The hole size in each plate was then measured and that width plotted as a function of the distance from the nozzle so that a profile of the jet cutting path could be drawn.

Profiles cut into the different pieces of wood showing the cutting power of the different jets

Figure 3. Profiles cut into the different pieces of wood showing the cutting power of the different jets as a function of distance and the actual amount of water flow as measured

As an additional part of the testing, a rough measure was kept of the effective nozzle life. Some other performance parameters for the different nozzles can be put into a table.

Performance of the different nozzles

Figure 4: Performance of the different nozzles

Clearly, just going out and buying the most expensive nozzle on the block is not necessarily the best idea. But it also depends on the use to which the nozzle is going to be applied. There are two different applications: that of cleaning a surface and that of cutting into it. The broader path achieved by nozzle 1, for example, which also removed the largest volume of wood per horsepower, makes it a good selection for cleaning and for reaching further from the nozzle as would be needed if one were cleaning the pipes of a heat exchanger bundle.

On the other hand, the more coherent flow through nozzle 2, which gave a narrower cut, might be a more effective tool in a cutting operation. In other cleaning operations, where the nozzle is being operated very close to the surface, then nozzle 3, which has a wider path, might be a better choice, though that is lost if the target surface is further away. And though there was not a great deal of difference in performance between nozzles 1 and 5, there is a considerable difference in price.

A smaller, lighter nozzle may be a beneficial trade-off if the nozzle body is fitting on the end of a lance that will be operated manually for several hours at a time.

There is an alternate way of using plywood as a target that I have also used in teaching class. The student is using a manually operated high-pressure cleaning gun at 10,000 psi and is to swing the gun horizontally so that the jet cuts into a piece of plywood that is set almost parallel with the jet path, but with the stream hitting the wood from the side initially further from the operator. But as the swing completes the jet cuts up where the nozzle almost touches it and then sweeps on past.

The result is that, over the distance, the jet can cut into the wood and a groove is carved into it.

Horizontal cuts into plywood

Figure 5. Horizontal cuts into plywood. There were about half-a-dozen students who had swiped the nozzle so that it just cleared the left edge of this 4-ft wide piece of plywood, and you may note that the cuts extend roughly ¾ of the way along the surface

Once the students had seen this cut, I would ask them how far away they thought, based on that measurement, the jet would cut into a person. Typically they said about three feet, and then, as a precaution, I suggested they add a foot or so more.

Then I took them over to a metal frame where we had hung a piece of pork. We carefully measured off the “safe” distance from the end of the nozzle to the pork.

“Now assume that is you”, I would say, “swing the jet as fast as you can, so that it barely has time to hit “your arm”, and we’ll just check that distance is correct.”

Piece of pork that has been traversed by a 10,000 psi jet several times

Figure 6. Piece of pork that has been traversed by a 10,000 psi jet several times, with a typical stand-off distance from the nozzle of more than four feet.

Invariably we got the result shown in Figure 6. The jet would cut into the meat to a typical depth of around two inches and groove the underlying bone. It was a salutary way of getting their attention about the safe use of waterjet technology, and I noticed that the staff also got a bit more cautious after we ran this class every year.

Waterjet Cutting – Introduction to testing waterjet nozzle performance

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

In the next few posts I will be writing about some of the tests that you can run to see how a nozzle is performing. But before getting into the details of the different tests, you should recognize that this is where a little homework will be required if you are to get the most benefit from the topic.

The world that encompasses waterjet use has grown beyond the simple categories by which we used to define it. New techniques make it possible to cut materials that used to be more difficult and expensive to produce, and as practical operational pressures have increased, so the scale, precision and economics of new opportunities have developed.

It is this range of applications that makes it impractical for me to give specific advice for every situation. So instead, by explaining how to make comparisons and what some benchmarks might be, I try to allow you to better understand your system, its capabilities and both the initial performance of nozzles. Hopefully, you’ll then be able to evaluate and decide when they may best be replaced.

One lesson I learned early was that nozzles from different companies behaved in different ways and that drawing conclusions on optimal performance, for example the selection for which pressure level and nozzle size was best, using one design would not necessarily hold with a competing design. Further, there were nozzles that began their life on our system doing very well relative to others, but which quickly declined in performance. Thus, as part of an evaluation of different designs, we would test the nozzle cutting performance against a standard requirement at fixed time intervals so that we would know when it was wearing out and should be replaced.

Change in the cutting depth of a jet stream at 50,000 psi when traversed over ASTM A108 steel

Figure 1. Change in the cutting depth of a jet stream at 50,000 psi when traversed over ASTM A108 steel as a function of the time that the nozzle had been in use.

Both the shape of the curve and the effective lifetimes of different competing nozzle designs varied quite significantly. And obviously, since most folk don’t spend a lot of their time cutting through more than an inch of steel, the operational lifetimes of nozzles will vary with the requirements for the particular job. Nevertheless, the relative ages at which nozzles can no longer reach that target can differ significantly.

Comparative effective nozzle life over which, operated at a pressure of 50,000 psi, a jet could cleanly cut a path through a 1.4 inch thick steel target at a traverse rate of 1.5 inches/minute.

Figure 2. Comparative effective nozzle life over which, operated at a pressure of 50,000 psi, a jet could cleanly cut a path through a 1.4 inch thick steel target at a traverse rate of 1.5 inches/minute.

As mentioned, the tests were carried out using nozzles from several manufacturers, and at the beginning of the test, the longest lasting nozzle was not necessarily the one that produced the fastest cut, but consistently over the interval and for about twice as long as the competition, it was able to achieve the goal.

Depths of cut in steel

Figure 3. Depths of cut in steel after (top) 1,000 minutes of nozzle use, and (bottom) after 1,500 minutes of nozzle use.

In the particular case in which we made the comparison, the major interest was in achieving a clean separation of the parts, and the edge quality was not as significant a factor. In many uses of this tool that edge quality will be important and would have given a different set of numbers (as Figure 3 would indicate) than the ones that were found for our application. As a result, the judgment that the nozzle is worn out will change to a different time, and the relative ranking of the different nozzle designs may also change.

The only way in which anyone can make a rational decision on which is the best nozzle for an application and how long it will be effective is by testing the nozzle against the stated requirement. When we began the test, we anticipated that the difference between nozzles from different manufacturers when fed with water at the same flow rate and with the same quantity and quality of abrasive would not differ that much. As Figure 2 shows, we were wrong in that idea.

There are a number of different impacts that a change in nozzle design (i.e. in most cases buying a competing design over that initially used) can bring to a cutting operation. However, these impacts are also governed by the pressure at which the work is being carried out, the amount of abrasive that is used, the relative nozzle diameters (if using a conventional abrasive waterjet system) and the speed at which the cut is made. But an initial assessment of relative merit should be carried out with equivalent parameters for the different designs.

In general, however, we ran tests at a number of pressures and with varying abrasive feed rates to ensure that the comparative evaluations were fair and consistent. As a result, we found that there were a number of different factors that came into play which are not always recognized and which could bias the results that we observed.

In the posts that follow this, I will first cover some of the different tests that can be used and then go on to explain some of the results and why they sometimes make it difficult to accept a simple comparison of results when, for example, the abrasive is not the same in both cases. To give a simple example of this, consider a conventional abrasive waterjet nozzle that is operated at increasing pressure.

Increasing the pressure will improve the cutting speed and/or the cut quality, as a general rule. It will reduce the amount of abrasive that is needed but this is where the “yes, but’s . . . .” start to appear. As the pressure of the jet increases, so the amount of abrasive that is broken within the mixing chamber will also increase so that the average size of the particle coming out of the nozzle will become smaller. The amount of this size reduction is a function of the quality of the abrasive that is being used and a function of the initial size of that abrasive.

Within a certain size range, that reduction in the particle size does not significantly change the cutting performance, but if the mix contains too many small particles, particularly if the distance to the work piece is also significant, then the cutting performance can be reduced because of the particle break-up. Different nozzle designs produce different amounts of very fine material even from the same feed rate of the same abrasive into the nozzle. When the initial feed rate of the abrasive or a different abrasive is used, estimating which design and set of operating pressures is best becomes more difficult as an abstract estimation.

This is why, in the posts that follow, the comparisons are made are based on actual measurements and why I recommend that everyone test their system using more than one design/set of operating parameters so that they can be confident that the combination that they are using will provide the best combination for the job to be done.